This invention relates to a valve operated by an electrical solenoid actuator, and more particularly to a proportional flow valve of this type.
Proportional flow valves find utility in performing mixing and measurement functions. For example, proportional flow valves are used to accurately blend different gasolines to achieve desired characteristics, such as particular octane ratings, and to mix hot and cold water to obtain a desired temperature. Also, a proportional valve may be used when it is desired to have a valve open gradually so that the flow of the controlled fluid begins slowly, after which the valve may be fully opened.
Typically, the power applied to the solenoid actuator is a periodically pulsed DC current, the amount of current varying with the length of "on" and "off" times of the pulses (sometimes referred to as pulse width modulation).
Prior art valves of the type described above which are of sufficient size to permit high rates of fluid flow are susceptible to wide variations in the force necessary to raise the valve member from the main valve seat from a fully closed position to an open one which make it difficult to precisely regulate low flow rates. Hence prior art valves suitable for controlling high flow rates are inherently unable to regulate fluid flow at low flow rates.
When a closed valve is being opened, as soon as the main seal of a prior art valve is lifted off of the main valve seat, there is a rush of fluid across the main valve seat which results in a rapid drop in pressure at the inlet port and, hence, above the main valve member which causes the main valve member to "jump" away from the main valve seat. Conversely, when an open valve is being closed, as the main valve seal nears the main valve seat, there is a sudden increase in pressure at the inlet port and, hence, above the main valve member which causes the main valve member to clamp down upon the main valve seat. The foregoing tendencies make it very difficult to precisely move the main valve member relative to the main valve seat to achieve constant low flow rates with the main valve seal spaced very closely to the main valve seat.
In addition to the foregoing problems of prior art valves, they are also subject to a tendency of the valve member to skew or tilt from the axis of the bore in the valve seat just before and after the valve is opened and closed. Random tilting of the main valve member affects the cross sectional area of flow from the inlet port through the opening in the main valve seat and, hence, the rate of flow. Consistency of flow rate at a predetermined level of solenoid engergizing current suffers when the main valve member is subject to skewing.